Laser desorption/ionization determination of molecular weight distributions of polyaromatic carbonaceous compounds and their aggregates

Abstract

Molecular weight distributions (MWDs) of model polyaromatic hydrocarbons (PAHs) and complex asphaltene samples have been investigated in laser desorption/ionization mass spectrometry (LDI-MS)experiments. Special efforts are devoted to the characterization of aggregation effects during the desorption process. It is found that non-covalent clusters of the PAHs and asphaltenes form readily in the desorbing plume. Aggregation is favoured in the experiments performed on dense samples at high laser energy and under continuous ion extraction conditions. In the absence of polar groups in the analyte molecules, the aggregation propensity correlates well with the size of the polycondensed system and with its degree of pericondensation, in qualitative agreement with previous theoretical predictions. For the polydispersed asphaltenes from two different crude oils, MWDs peaking at masses smaller than 500 amu with a highmass tail extending up to about 3000 amu have been observed, yielding average weights around 900 amu. Such MWDs are in good agreement with previous mass spectrometric measurement, as well as with diffusion studies in solution. In addition, stable asphaltene aggregates have been detected giving rise to two broad bands in the mass spectrum corresponding to average molecular weights of 2200-3100 amu and 15 000-19 000 amu, respectively. It is concluded that the strong aggregation propensity of asphaltenes is likely to be responsible for the apparent inconsistency between the MWD for these compounds determined by different groups in independent LDI-MS experiments. The reliability of different sample preparation procedures, including solvent-free methods, is discussed, and strategies are outlined that serve to apply the potentiality of LDI mass spectrometry to the characterization of covalent and non-covalent compounds in complex carbonaceous systems.

Review of the Molecular Structure and Aggregation of Asphaltenes and Petroleomics

“…Some matrix-assisted laser desorption results are consistent with all other mass spectral techniques (Miller et al 1998). Very recently, it has been conclusively established that LDI MS can give rise to gas-phase aggregation if (1) too high a laser power is used, (2) too high a surface concentration of asphaltenes is used (or not), and (3) if fast ion extraction is used (Hortal et al 2006;Martinez-Haya et al 2007a, b). Fig.…”

Photoionisation and photodissociation studies of nonvolatile organic molecules by synchrotron VUV photoionisation mass spectrometry and theoretical calculations

“…To date, numerous experimental methods have been utilised to measure the molecular weight distribution of heavy oil, from which the mean molecular weight can be deduced. However, pronounced controversy arose from these different techniques [124][125][126][127][128][129]. We measured the photoionisation mass spectra of four samples of heavy oil using IR LD/ VUV PIMS at the photon energy of 9.8 eV [58], and typical peaks of hydrocarbon in saturates were observed.…”

Characterization of Tars from the Thermal Processing of Baganuur and Tavan Tolgoi Coals from Mongolia, Using SEC, UV-F, IR and Mass Spectrometry

“…Another problem with laser desorption is the formation of ionized clusters of molecules when using too high laser fluences or thick sample layers. Hortal et al [15,16] and Martinez-Haya et al [17] showed that the LD-MS spectrum of an asphaltene depended on the concentration of species in the sample and on the laser fluence applied. The thickness of sample applied to the target plate was a factor in the formation of aggregates or clusters of molecules in the reactive gas plume formed by the laser shot; thin layers were less likely to form cluster ions than thick layers.…”